TASK 31: SceneGraph component + baked-crust compositor optimization

- Add ElementKind (Static/Dynamic) to SceneElement base; Source/WebcamSceneConfig classify
- Services/SceneGraph.cs: owns the Scenes collection (ViewModel's Scenes delegates to it),
  the element mutation surface (Add/Insert/Remove/Move, each invalidating the bake), and the
  queries that were scattered LINQ (GetBackground/GetWebcam/GetChatBoxes/GetSplitPoint/IsStatic)
- SceneCompositor: split-aware BakeStaticBase + CompositeLayers + Render(.., staticBase, split);
  builds/caches the static base below the split point in source-rect space
- FramePump: optional SceneGraph -> optimized bake+composite path; falls back to full render
- MainViewModel: routes element mutations through the graph; invalidates the bake on static
  layout/opacity/visibility/useDefaultBackground changes and after background heal/Ensure
- Integration test SceneGraphTests.BakedStaticBase_WithDynamicLayer_CompositesCorrectly

Derivative survey (mandated): tried before writing — OBS does per-source opacity/visibility
caching and static-scene baking; this mirrors OBS's 'cached static source' optimization.
3 documented defensive deviations from the spec (ChatOverlayLayer stays decoupled;
background helpers stay VM-static for direct testability; full facade peel deferred post-1.0)
in TASKS.md + ai.md. Scope check passed; clean build 0 warnings.

Models/Source.cs

@@ -63,6 +63,11 @@ public class Source : SceneElement
 
     public override bool IsDraggable => Type is SourceType.Image or SourceType.WebSource or SourceType.ChatBox;
 
+    /// <summary>Live captures, chat, web sources and media are dynamic; static art is fixed.</summary>
+    public override ElementKind Kind => Type is SourceType.DisplayCapture or SourceType.WindowCapture
+        or SourceType.ChatBox or SourceType.WebSource or SourceType.MediaSource
+        ? ElementKind.Dynamic : ElementKind.Static;
+
     public override bool IsWebSource => Type == SourceType.WebSource;
 
     /// <summary>The permanent scene background (live desktop/game capture). Never removable/reorderable.</summary>